I wonder, is it even worth my doing research on charities, when there exist resources like givewell, that will almost certainly be able to do a more thorough and more accurate analysis than I would be able to do?

If you have very similar values to the folks at GiveWell, then I would advise you to simply review their research and donate to their top-ranked charities, rather than conducting your own research, given that your time is valuable. If you have somewhat different values from the folks at GiveWell, you might look into organizations (MIRI, Animal Charity Evaluators, etc.) who have already conducted effective altruism-relevant research in other fields, before branching off and starting your own research.

I may value the well being of non-human mammals, relative to human mammals, more than others, and so chose to support animal rights groups

Animal Charity Evaluators is a Givewell-like charity evaluator which has its roots in the EA movement, and focuses on evaluating organizations which seek to reduce non-human suffering. I'm not as familiar with them as I am with GiveWell, but the fact that Animal Charity Evaluators branched off of 80,000 Hours is a good signal of their credibility.